In recent years, along with the miniaturization and integration of semiconductor integrated circuits, a semiconductor exposure device (hereinafter referred to as “exposure device”) has been required to have higher resolution. For this reason, shortening of the wavelength of light that is emitted from an exposure light source has been under development. Generally, as an exposure light source, a gas laser apparatus is used instead of a conventional mercury lamp. For example, as a gas laser apparatus for exposure, a KrF excimer laser apparatus configured to output ultraviolet laser beam with a wavelength of 248 nm as well as an ArF excimer laser apparatus configured to output ultraviolet laser beam with a wavelength of 193 nm may be used.